1. Field of the Invention
The invention relates to a PFC converter and more particularly to an internal power supply circuit for buck-boost power factor correction (PFC) converter.
2. Description of the Related Art
PFC converters are utilized to improve the power factor of AC power. An important consideration in the design of power converters is that a power converter should provide a high power factor. The power factor of power converters generally refers to the ratio of the true power to the product of the voltage and current in the circuit. A high power factor is one which approximates or exceeds 0.9, with the maximum power factor being 1.0. For example, in an electric powered device, a load with a low power factor draws more current than a load with a high power factor for the same amount of useful power transferred.
Solid-state PFC controllers are also known and have been designed for each of the principal switch-mode power converter topologies: buck, boost, and buck-boost. In general, buck-derived circuits, including buck-boost circuits, interrupt the line input current, while boost-derived circuits do not.
Buck-boost power converters are also generally well-known in the art: a buck (step-down) converter followed by a boost (step-up) converter. The output voltage is of the same polarity as the input, and can be lower or higher than the input. Such a non-inverting buck-boost converter may use a single inductor that is used as both the buck inductor and the boost inductor.
Such conventional buck-boost power converters include an internal power supply circuit that usually consists of an auxiliary winding of PFC inductor, a charge pump circuit and a linear voltage regulator. A major limitation of such internal power supply circuits in the buck-boost PFC converters is that the voltage after the charge pump includes a high ripple voltage with a frequency of double main input frequency. A consequence of this high ripple voltage is that it causes the linear voltage regulator to have a high power loss and thus to be less cost effective.
A need exists for a power factor correction circuit that avoids limitations of prior art circuits. In particular, a need exists for an internal power supply circuit for the buck-boost PFC converter that eliminates or reduces the ripple voltage to allow the voltage after the linear voltage rectifier to be linear to DC bus voltage (output of the buck-boost PFC converter) at steady state. A smaller or eliminated voltage ripple will improve the power loss factor in linear voltage regulator and allow the buck-boost PFC converter to be more cost effective.
The present invention addresses such needs.